Conventional intruder systems operate by means of sensors deployed outside or within a building. These technologies include passive infrared (PIR) sensors, microwave sensors, ultrasonic sensors, magnetic reed switch sensors, vibration sensors, and seismic sensors. A passive infrared detector, for example, may include pixel pairs wired as opposite inputs to a differential amplifier. Detection of an IR source causes closing of a relay switch, and issuing an alarm signal.
In a typical application, the particular sensor is used to detect and activate a corresponding relay that is wired into an alarm panel. The alarm panel transmits the alarm event, typically over the public switched telephone network (PSTN), to a central monitoring station. The difficulties these sensors can have are the ability to cope well with variable environmental conditions within the monitored area, and the ability to distinguish between a valid alarm and a false alarm.